The primary goals of the proposed research are: (1) to discern the way in which Toxoplasma gondii causes destructive lesions of the eye; (2) to study the mechanisms of host defense against the parasite;; (3) to analyze the immunologic responses of the host, separating those phases of hypersensitivity which might be helpful from those which might be harmful to the eye; (4) to provide new modes of therapy for ocular toxoplasmosis, based upon augmentation of the host's natural immunologic responses or upon trials of new chemotherapeutic agents in animal models; and (5) to discern the causes of recurrent lesions in ocular toxoplasmosis. Attempts will be made to analyze the antigens of Toxoplasma, exploiting the new knowledge gained in this area for the development of laboratory tests that will be helpful in assessing the immunologic capabilities of the host. This, in turn, may provide information about the variability of patient responses to infection and may ultimately point the way to efficient immunization procedures for segments of the population that are thought to be at the highest risk. Bibliographic references: O'Connor, G.R. Ocular toxoplasmosis. Jap. Journal Ophthal. 19:1-24, 1975; O'Connor, G.R. Basic mechanisms responsible for the initiation and perpetuation of anterior segment inflammation. Trans. Amer. Acad. Ophthal. Otolarngol. 79:56-61, 1975.